I asked Larry Ball, author of Tom Horn: In Life and Legend and he replied, “I am not aware that any handwriting comparison has ever been made. In fact, I have never seen one of the notes and am not aware that any note is still extant. Should one turn up, it would be a simple matter to compare it with Horn’s existing correspondence.”
Marshall Trimble is Arizona’s official historian and vice president of the Wild West History Association. His latest book is Arizona Outlaws and Lawmen; The History Press, 2015. If you have a question, write: Ask the Marshall, P.O. Box 8008, Cave Creek, AZ 85327 or email him at firstname.lastname@example.org.